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Photophysics of metal-organic π-conjugated oligomers and polymers
ELSEVIER
Synthetic
Metals
102 (1999)
1585-1586
Photophysics of metal-organic z-conjugated oligomers and polymers Department
of
K. D. Ley, K. A. Walters, K. S. Schanze* Chemistry, University of Florida, P. 0. Box 117200, Gainesville,
FL
3261 I-7200
Abstract A series of phenylene-ethynylene based n-conjugated oligomers that contain a 2,2’-bipyridine metal chelating unit has been synthesized by using Pd-mediated coupling chemistry. The photophysics of the free oligomers and complexes of the oligomers with the Re’(CO)sCl metal chromophore is reported. These oligomers serve as excellent models for n-conjugated metal-organic polymers. Keywords: other conjugated
ador
conducting polymers; optical absorption
and emission spectroscopy; photoluminescence.
1. Introduction There has been increasing interest in the photophysics of n-conjugated polymers that contain photoactive transition metal complex chromophores interspersed along the polymer backbone [l-6]. Introduction of metal complex chromophores into n-conjugated polymers allows systematic tuning of the polymer bandgap and may impart other useful properties such as improved photoconductivity or non-linear optical response. We recently synthesized a series of n-conjugated polymers that contain the (2,2’-bipyridine)Re’(CO)sCl chromophore incorporated into the n-conjugated backbone [3]. These metal-organic polymers feature a red-shifted absorption band and unusual long-wavelength luminescence that has tentatively been assigned to a metal-to-ligand charge transfer (MLCT) transition arising from a dx (Re) + n* (bpy-polymer) transition. Now, in an effort to better understand the optical properties of the polymers, we have synthesized a series of oligomers that consist of the basic repeat structure of the metalbipyridine containing polymers. In the present report we provide a preliminary summary of the photophysical properties of these oligomers. 2. Structures
2 :M =.
PO
Figure
and Synthesis
Absorption
Spectroscopy
The absorption spectra of oligomers 1 - 4 and polymer PO in THF solution are compared in Figure 2a. All of the spectra exhibit two bands in the the 300 - 500 nm region. The lowest energy band is assigned to the long-axis polarized n,n* (HOMO + LUMO) transition and the second band is due to the short-axis polarized rc,rc* transitions. The oscillator strength of both bands increases as the conjugation length of the oligomers increase. The low-energy absorption band red-shifts considerably from 1 to 2, but the position and bandshape of this 0379-6779/99/$ - see front Pll: SO379-6779(98)01041-S
matter
0 1999 Elsevier
Science
S.A.
All rights
R = “C,,H,,
PI :x
R = n-C&,
= 2. M = Re’(CO),CI,
1. Structures of oligomers
M, = 26 kD
R = n-C18H3,
and polymers.
transition is relatively constant in 2 - 4, indicating that the bandgap of the oligomers is defined very early in the series. Interestingly, the absorption spectrum of polymer PO is very similar to that of oligomer 4, except that the low-energy transition is slightly red-shifted in the oligomer. The red-shift is likely due to the effect of the bipyridine chromophore on the bandgap of the oligomer. Comparison of the spectra of Re-1 - Re-4 (Fig. 2b) with those of the corresponding free oligomers reveals that metallation induces a red-shift of the lowest n,rc* absorption. The oscillator strength of the low energy band is large (E = 75 90 rnM’cm-‘) but remains relatively constant throughout the series of oligomers. This band is due to the lowest ~,rt* (longaxis polarized) transition of the metallated oligomer. Moreover, the transition likely originates from a chromophore defined by the bis-(dialkoxyphenylethynyl)-capped bipyridine segment of the oligomer. The red-shift occurs because metallation forces the bipyridine segment into a planar conformation; by contrast, in the free oligomers the bipyridine is twisted out of planarity. An important point is that the dn (Re) -+ rc* MLCT absorption, which is expected to arise in the 400 - 450 nm region [7], is buried under the considerably more intense X,TC* feature. Another feature of note is the remarkable similarity in the absorption spectra of Re-4 and Pl. This similarity suggests that the oligomer provides a reasonable model for the optical properties of the metal-organic polymers.
The structures of the oligomers and polymers are shown in Figure 1. The conjugated oligomers were synthesized by an iterative sequence involving Sonogashira coupling of appropriately protected and functionalized aryl iodides and terminal acetylenes. Metallation of 1 - 4 was accomplished by heating the oligomers with Re’(CO)sCl in toluene. Polymers PO and Pl were prepared by AA + BB polymerization of diiodo- and diethynyl-substituted arenes (or telomers). 3. UV-Visible
M,.37kO
~~2. M = Rd(co),a
reserved.
1586
KS. Schanze 300
,
et al. I Synthetic
,
PO -
60
a : -60
cI a
- 40
; 1.
- 20
: 1.
0.6
102 (1999)
1585-1586
excellent models for metal-bipyridine containing phenyleneethynylene and phenylene-vinylene polymers [l-6]. The absorption data for the metal-organic oligomers reveals that the lowest energy absorption is due to a bipyridine-based n,n* transition- not to a MLCT transition as had been previously suggested for the related metal-organic polymers [l-3]. At lowtemperatures 3MLCT photoluminescence is observed from the metal-organic oligomers. Studies in progress seek to fully characterize the photophysics of these novel metal-organic polymers. This work was supported by a grant from the U.S. National Science Foundation (Grant No. CHE-9401620).
1.0 0.8
Metals
Jl z D, f
I
’
I
(D 0.4
300
400 Wavelength
D CT
500 I nm c -
Figure 2. UV-visible absorption spectra for THF solutions. (a) Oligomers 1 - 4, scale at left. Dotted line is spectrum of polymer PO; scale for PO is at right (E value calculated by using polymer repeat unit concentration). (b) Metal-organic oligomers Re-1 - Re-4, scale at left. Dotted line is spectrum of polymer Pl; absolute absorptivity not determined for this sample. 4. Photoluminescence Oligomers
Spectroscopy
of
Metal-Organic
Fluorescence is observed from all of the oligomers, but due to space limitations in the present report we describe only the low-energy MLCT emission. At low temperature transition metal complexes of the type (NN)Re’(CO)&l typically luminescence from a 3MLCT state [7]. By analogy, we expect 3MLCT emission from oligomers Re-1 - Re-4. Figure 3 illustrates the photoluminescence spectra of the metal-organic oligomers obtained at 80 K in a 2-methyl tetrahydrofuran solvent glass. Complex Re-1, which has the shortest conjugation length, exhibits a highly structured emission with h,, = 590 nm and lifetime (~~3 of 6.2 ps. Assignment of this emission to a 3MLCT state is supported by the observation of a remarkably similar structured, but red-shifted emission from the complex (bpy)2Ru”(l)2’ (h max = 650 nm). (A red-shift is expected for an MLCT transition because Ru” has a lower oxidation potential than Re’). MLCT emission is also observed from Re-2, Re-3 and Re-4 at low temperature (TV,, = 2.5, 5.1 and 7.6 ps, respectively). The band-shape of the emission from the longer oligomers is considerably more complex than that of Re-1. This may arise because in the glass the dialkoxyphenylene units are frozen in a range of different conformations giving rise to molecules with different effective conjugation lengths. 5. Summary
I
500
550
600 Wavelength
650
700
750
800
I nm
Figure 3. Photoluminescence spectra of metal-organic oligomers in 2-MTHF solution at 80 K with 440 nm excitation. (a) Re-1; (b) Re-2; (c) Re-3; (d) Re-4. References [1] [2]
Z. Peng, L. Yu, J. Am. Chem. Sot. 118 (1996) 3777. Z. Peng, A. R. Gharavi, L. Yu, J. Am. Chem. Sot. 119 (1997)
4622.
[3]
K. D. Ley, C. E. Whittle, M. D. Bartberger, Schanze, J. Am. Chem. Sot. 119 (1997) 3423.
K. S.
[4]
S. Tokura, T. Yasuda, Y. Segawa, M. Kira, Chem. Lett. (1997) 1163.
[5]
P. K. Ng, X. Gong, W. T. Wong, W. K. Chan, Macromol. Rapid Commun. 18 (1997) 1009.
[6]
W. Y. Ng, W. K. Chan, Adv. Mater. 9 (1997) 716.
[7]
L. A. Worl, R. Duesing, P. Chen, L. Della Ciana, T. J. Meyer, J. Chem. Sot. Dalton Trans. (1991) 3423.
and Acknowledgments
A series of conjugated metal-organic phenyleneethynylene oligomers has been synthesized and preliminary photophysical data is reported. The oligomers provide
Synthetic
Metals
102 (1999)
1585-1586
Photophysics of metal-organic z-conjugated oligomers and polymers Department
of
K. D. Ley, K. A. Walters, K. S. Schanze* Chemistry, University of Florida, P. 0. Box 117200, Gainesville,
FL
3261 I-7200
Abstract A series of phenylene-ethynylene based n-conjugated oligomers that contain a 2,2’-bipyridine metal chelating unit has been synthesized by using Pd-mediated coupling chemistry. The photophysics of the free oligomers and complexes of the oligomers with the Re’(CO)sCl metal chromophore is reported. These oligomers serve as excellent models for n-conjugated metal-organic polymers. Keywords: other conjugated
ador
conducting polymers; optical absorption
and emission spectroscopy; photoluminescence.
1. Introduction There has been increasing interest in the photophysics of n-conjugated polymers that contain photoactive transition metal complex chromophores interspersed along the polymer backbone [l-6]. Introduction of metal complex chromophores into n-conjugated polymers allows systematic tuning of the polymer bandgap and may impart other useful properties such as improved photoconductivity or non-linear optical response. We recently synthesized a series of n-conjugated polymers that contain the (2,2’-bipyridine)Re’(CO)sCl chromophore incorporated into the n-conjugated backbone [3]. These metal-organic polymers feature a red-shifted absorption band and unusual long-wavelength luminescence that has tentatively been assigned to a metal-to-ligand charge transfer (MLCT) transition arising from a dx (Re) + n* (bpy-polymer) transition. Now, in an effort to better understand the optical properties of the polymers, we have synthesized a series of oligomers that consist of the basic repeat structure of the metalbipyridine containing polymers. In the present report we provide a preliminary summary of the photophysical properties of these oligomers. 2. Structures
2 :M =.
PO
Figure
and Synthesis
Absorption
Spectroscopy
The absorption spectra of oligomers 1 - 4 and polymer PO in THF solution are compared in Figure 2a. All of the spectra exhibit two bands in the the 300 - 500 nm region. The lowest energy band is assigned to the long-axis polarized n,n* (HOMO + LUMO) transition and the second band is due to the short-axis polarized rc,rc* transitions. The oscillator strength of both bands increases as the conjugation length of the oligomers increase. The low-energy absorption band red-shifts considerably from 1 to 2, but the position and bandshape of this 0379-6779/99/$ - see front Pll: SO379-6779(98)01041-S
matter
0 1999 Elsevier
Science
S.A.
All rights
R = “C,,H,,
PI :x
R = n-C&,
= 2. M = Re’(CO),CI,
1. Structures of oligomers
M, = 26 kD
R = n-C18H3,
and polymers.
transition is relatively constant in 2 - 4, indicating that the bandgap of the oligomers is defined very early in the series. Interestingly, the absorption spectrum of polymer PO is very similar to that of oligomer 4, except that the low-energy transition is slightly red-shifted in the oligomer. The red-shift is likely due to the effect of the bipyridine chromophore on the bandgap of the oligomer. Comparison of the spectra of Re-1 - Re-4 (Fig. 2b) with those of the corresponding free oligomers reveals that metallation induces a red-shift of the lowest n,rc* absorption. The oscillator strength of the low energy band is large (E = 75 90 rnM’cm-‘) but remains relatively constant throughout the series of oligomers. This band is due to the lowest ~,rt* (longaxis polarized) transition of the metallated oligomer. Moreover, the transition likely originates from a chromophore defined by the bis-(dialkoxyphenylethynyl)-capped bipyridine segment of the oligomer. The red-shift occurs because metallation forces the bipyridine segment into a planar conformation; by contrast, in the free oligomers the bipyridine is twisted out of planarity. An important point is that the dn (Re) -+ rc* MLCT absorption, which is expected to arise in the 400 - 450 nm region [7], is buried under the considerably more intense X,TC* feature. Another feature of note is the remarkable similarity in the absorption spectra of Re-4 and Pl. This similarity suggests that the oligomer provides a reasonable model for the optical properties of the metal-organic polymers.
The structures of the oligomers and polymers are shown in Figure 1. The conjugated oligomers were synthesized by an iterative sequence involving Sonogashira coupling of appropriately protected and functionalized aryl iodides and terminal acetylenes. Metallation of 1 - 4 was accomplished by heating the oligomers with Re’(CO)sCl in toluene. Polymers PO and Pl were prepared by AA + BB polymerization of diiodo- and diethynyl-substituted arenes (or telomers). 3. UV-Visible
M,.37kO
~~2. M = Rd(co),a
reserved.
1586
KS. Schanze 300
,
et al. I Synthetic
,
PO -
60
a : -60
cI a
- 40
; 1.
- 20
: 1.
0.6
102 (1999)
1585-1586
excellent models for metal-bipyridine containing phenyleneethynylene and phenylene-vinylene polymers [l-6]. The absorption data for the metal-organic oligomers reveals that the lowest energy absorption is due to a bipyridine-based n,n* transition- not to a MLCT transition as had been previously suggested for the related metal-organic polymers [l-3]. At lowtemperatures 3MLCT photoluminescence is observed from the metal-organic oligomers. Studies in progress seek to fully characterize the photophysics of these novel metal-organic polymers. This work was supported by a grant from the U.S. National Science Foundation (Grant No. CHE-9401620).
1.0 0.8
Metals
Jl z D, f
I
’
I
(D 0.4
300
400 Wavelength
D CT
500 I nm c -
Figure 2. UV-visible absorption spectra for THF solutions. (a) Oligomers 1 - 4, scale at left. Dotted line is spectrum of polymer PO; scale for PO is at right (E value calculated by using polymer repeat unit concentration). (b) Metal-organic oligomers Re-1 - Re-4, scale at left. Dotted line is spectrum of polymer Pl; absolute absorptivity not determined for this sample. 4. Photoluminescence Oligomers
Spectroscopy
of
Metal-Organic
Fluorescence is observed from all of the oligomers, but due to space limitations in the present report we describe only the low-energy MLCT emission. At low temperature transition metal complexes of the type (NN)Re’(CO)&l typically luminescence from a 3MLCT state [7]. By analogy, we expect 3MLCT emission from oligomers Re-1 - Re-4. Figure 3 illustrates the photoluminescence spectra of the metal-organic oligomers obtained at 80 K in a 2-methyl tetrahydrofuran solvent glass. Complex Re-1, which has the shortest conjugation length, exhibits a highly structured emission with h,, = 590 nm and lifetime (~~3 of 6.2 ps. Assignment of this emission to a 3MLCT state is supported by the observation of a remarkably similar structured, but red-shifted emission from the complex (bpy)2Ru”(l)2’ (h max = 650 nm). (A red-shift is expected for an MLCT transition because Ru” has a lower oxidation potential than Re’). MLCT emission is also observed from Re-2, Re-3 and Re-4 at low temperature (TV,, = 2.5, 5.1 and 7.6 ps, respectively). The band-shape of the emission from the longer oligomers is considerably more complex than that of Re-1. This may arise because in the glass the dialkoxyphenylene units are frozen in a range of different conformations giving rise to molecules with different effective conjugation lengths. 5. Summary
I
500
550
600 Wavelength
650
700
750
800
I nm
Figure 3. Photoluminescence spectra of metal-organic oligomers in 2-MTHF solution at 80 K with 440 nm excitation. (a) Re-1; (b) Re-2; (c) Re-3; (d) Re-4. References [1] [2]
Z. Peng, L. Yu, J. Am. Chem. Sot. 118 (1996) 3777. Z. Peng, A. R. Gharavi, L. Yu, J. Am. Chem. Sot. 119 (1997)
4622.
[3]
K. D. Ley, C. E. Whittle, M. D. Bartberger, Schanze, J. Am. Chem. Sot. 119 (1997) 3423.
K. S.
[4]
S. Tokura, T. Yasuda, Y. Segawa, M. Kira, Chem. Lett. (1997) 1163.
[5]
P. K. Ng, X. Gong, W. T. Wong, W. K. Chan, Macromol. Rapid Commun. 18 (1997) 1009.
[6]
W. Y. Ng, W. K. Chan, Adv. Mater. 9 (1997) 716.
[7]
L. A. Worl, R. Duesing, P. Chen, L. Della Ciana, T. J. Meyer, J. Chem. Sot. Dalton Trans. (1991) 3423.
and Acknowledgments
A series of conjugated metal-organic phenyleneethynylene oligomers has been synthesized and preliminary photophysical data is reported. The oligomers provide